Soaking, Sprouting, Fermenting: What Processing Does for Plant Protein

Why plant protein needs processing

Many plants protect themselves with compounds that slow down digestion. Herreman et al. 2020 list trypsin inhibitors, phytate, saponins, glycoalkaloids and glucosinolates. On top of that come fiber and the structure of the proteins themselves. Moughan & Lim 2024 write that processing such as soaking, heating, extraction or extrusion often improves digestibility, especially in plant foods, because it neutralizes such compounds.

An important note from Hertzler et al. 2020: antinutrients are not only bad. In small amounts, some of them can, for example, lower blood sugar or cholesterol.

Heating: the biggest effect

The clearest figure comes from an FAO report (FAO sub-committee 2012). Raw soybean meal with active trypsin inhibitors was tested in pigs, once raw and once autoclaved:

So heating roughly doubled digestibility. This shows why cooking legumes matters so much. More on this in the article Cooking, frying, roasting.

Isolation: fewer inhibitors, but not automatically better

Protein isolates and concentrates are made in a way that removes most of the antinutrients. According to Hertzler et al. 2020, soy protein isolate is 96 % or more digested, soy flour only 84 %.

The pig study by Mathai et al. 2017 essentially confirms this: the isolate contained only about a third of the trypsin inhibitors of the soybean meal (2.75 versus 8.06 units per mg), and almost all essential amino acids were digested better (on average 96 % versus 93 %). Even so, the soybean meal had the higher DIAAS: 105 versus 98 for adults. So better digestibility alone doesn't settle it. The amino acid pattern counts too.

Extrusion: higher values for pea protein

In extrusion, a mass is pressed through a die under pressure, heat and shear. This is how protein snacks or the fibrous texture of meat substitutes are made. According to Herreman et al. 2020, pea protein reaches an average DIAAS of 70. After extrusion, individual measurements came in at 82 or 86, depending on temperature (young child pattern (0.5–3 years)).

It's not quite that simple, though. In their lab study on meat substitutes, Alotaibi et al. 2026 write, citing earlier work, that isolation, extrusion and heating can also reduce protein quality. More on this in the article Plant-based chicken vs. chicken.

Soaking, sprouting, fermenting: plausible, but barely measured

For these classic kitchen methods, the data in the studies reviewed is thin:

• Soaking: Hertzler et al. 2020 name it as one way to reduce antinutrients. Herreman et al. 2020 cite a study on faba beans in which phytate and trypsin inhibitors dropped most effectively when the beans were first soaked, then dehulled and then heated. They give no figures on protein quality.
• Sprouting: Hertzler et al. and Sharma et al. 2025 list it as a method for breaking down antinutrients. Measured values for protein quality are missing.
• Fermenting: Herreman et al. cite a study in which fermentation or enzyme treatment of soy reduced antinutrients and increased digestibility in young pigs. The text gives no figures for this. Sharma et al. mention improved digestibility in the lab for soy residues.

For tempeh, miso or sourdough, there is no DIAAS measurement in our source base. Anyone quoting specific figures here should be able to name a source.

When processing does harm

Moughan & Lim 2024 stress that processing does not always improve digestibility. Heating and drying in particular can destroy lysine through the Maillard reaction, including during storage. Examples from the studies:

• Soybean meal and rapeseed meal lost 30 to 40 % of their reactive lysine after toasting with sugar (Herreman et al. 2020).
• In tofu, methionine and cysteine were digested less well than in soy milk. The authors suspect that coagulation is responsible. The DIAAS for adults dropped from 117 to 97, measured in minipigs (Reynaud et al. 2021).
• For wheat, Mathai et al. 2017 suspect that the low digestibility of lysine and tryptophan is due to heat damage during drying or milling.

Limitations

• Much of the data comes from animal feed research, often with raw ingredients. Herreman et al. themselves write that these do not reflect the processed and cooked forms of human foods.
• The extrusion values for pea are in the study's appendix and are based on individual data sets.
• The review by Sharma et al. relies mostly on other reviews. We only use it for general statements.

Frequently asked questions

Sources

1. Moughan, P. J. & Lim, W. X. J. (2024): Digestible indispensable amino acid score (DIAAS): 10 years on. Frontiers in Nutrition 11, 1389719. doi:10.3389/fnut.2024.1389719
2. Hertzler, S. R., Lieblein-Boff, J. C., Weiler, M. & Allgeier, C. (2020): Plant proteins: assessing their nutritional quality and effects on health and physical function. Nutrients 12, 3704. doi:10.3390/nu12123704
3. Herreman, L., Nommensen, P., Pennings, B. & Laus, M. C. (2020): Comprehensive overview of the quality of plant- and animal-sourced proteins based on the digestible indispensable amino acid score. Food Science & Nutrition 8, 5379–5391. doi:10.1002/fsn3.1809
4. Mathai, J. K., Liu, Y. & Stein, H. H. (2017): Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS). British Journal of Nutrition 117, 490–499. doi:10.1017/S0007114517000125
5. Moughan, P. J., Gilani, S., Rutherfurd, S. M. & Tomé, D. (2012): Report of a Sub-Committee of the 2011 FAO Consultation on “Protein Quality Evaluation in Human Nutrition”, with appendix by Moughan & Rutherfurd: True ileal amino acid and protein digestibility for selected human foods. FAO, Rome.
6. Reynaud, Y., Buffière, C., Cohade, B. et al. (2021): True ileal amino acid digestibility and digestible indispensable amino acid scores (DIAASs) of plant-based protein foods. Food Chemistry 338, 128020. doi:10.1016/j.foodchem.2020.128020
7. Alotaibi, M., Muleya, M., Salter, A., Hoad, C. & Eldeghaidy, S. (2026): Comparison of the protein quality of chicken and plant-based chicken analogues using the INFOGEST in-vitro digestion system. Food Chemistry 520, 149734. doi:10.1016/j.foodchem.2026.149734
8. Sharma, K., Zhang, W. & Rawdkuen, S. (2025): Dietary plant-based protein supplements: sources, processing, nutritional value, and health benefits. Foods 14, 3259. doi:10.3390/foods14183259

Values for individual foods and where they come from are listed under Data & sources. How proteinscore calculates is explained under Method.